Magnetostriction occurs when a material on exposure to a magnetic field develops significant strain: at room temperature, sample dimensions can change by as much as fractions of a percent. Conversely, the straining of a magnetostrictive material changes its magnetization state.
Magnetostrictive materials have been used with electromagnetic actuators to form transducers which serve as, for example, ultrasonic generators or fine control valves for the metering of fluids. In these applications, variation of the magnetic field is employed to produce varying strains in the magnetostrictive material to produce a mechanical output. Conversely, a suitable magnetostrictive material might be employed as a torque or force sensor. In fact, such materials are being considered as torque sensors in the form of a magnetostrictive ring mounted on a shaft such as an automobile steering shaft. Torque in such a shaft would strain the magnetostrictive ring, giving rise to a detectable change in the ring's axial magnetization.
Maximizing device performance naturally suggests using materials having large saturation magnetostriction, .lambda..sub.s, which is a dimensionless measure of the field-induced strain frequently expressed in units of parts per million (ppm). Extremely high values of .lambda..sub.s are found in rare earth-iron compounds such as the terbium-iron compound, TbFe.sub.2, where .lambda..sub.s equals 1750 ppm for a polycrystalline sample. Unfortunately, the rare earth-iron compounds are very brittle materials having little tensile strength, an unpropitious characteristic for automotive applications requiring good mechanical properties. On the other hand, stronger and tougher materials such as steels have very limited magnetostriction: T250 maraging steel, which is currently being evaluated in torque sensors, has a .lambda..sub.s of only .about.30 ppm. The wide gulf between these two extremes offers ample opportunity and challenge for developing new magnetostrictors combining good magnetostriction with satisfactory mechanical properties.
The prospect of embedding magnetostrictive powder in a strengthening matrix has been sporadically explored as follows.
The Clark and Belson patent, U.S. Pat. No. 4,378,258, entitled "Conversion Between Magnetic Energy and Mechanical Energy," reported sintering cold-pressed pellets of ErFe.sub.2 with nickel and TbFe.sub.2 with iron. Few details of the properties of these materials were provided. They retain some magnetostriction, but as it turns out, the sintered bodies are brittle and of insufficient strength for many applications such as automotive sensor applications. Clark and Belson also produced resin-bonded composites of the RE-Fe.sub.2 (RE=rare earth) magnetostrictive compounds, but these materials also are of insufficient mechanical strength for automotive applications.
Peters and Huston of the International Nickel Company attempted to prepare composites of SmFe.sub.2 in nickel by sintering, by extrusion and by hot pressing, but they obtained values of magnetostriction which were only modestly larger than that of the nickel alone and did not recommend the practices. See D. T. Peters and E. L. Huston, "Nickel Composite Magnetostrictive Material Research for Ultrasonic Transducer," January 1977, Naval Electronic Systems Command Contract No. N00039-76-C-0017, US Department of Commerce National Technical Information Service, ADA 040336; and D. T. Peters, "Production and Evaluation of ReF(2)-Nickel Composite Magnetostrictive Materials," Final Report, January 1979, Naval Electronic Systems Command Contract No. N00039-77-C-0108, US Department of Commerce National Technical Information Service, ADA 066947.
Others have also made magnetostrictive composites of RE-Fe.sub.2 materials in epoxy binders. However, none of the above-referenced attempts have produced composites of suitable magnetostriction and mechanical properties to serve as, for example, torque sensors in demanding environments such as automotive applications.
An example of a torque sensor such as might be used in an automotive application is found in I. J. Garshelis, IEEE Trans. Magn. 28, 2202 (1992). Garshelis describes a magnetostrictive ring in which circumferential magnetization is maintained by a large static hoop stress, the hoop stress also serving to rigidly attach the ring to the shaft carrying the torque. Stresses in the ring associated with the torque tilt the magnetization away from the circumferential direction. An axial component of magnetization develops and in turn produces a magnetic field in the space around the ring which is detected by a magnetic field intensity sensor; the magnetic field intensity is used to measure the torque in the shaft. An example of the application of such a shaft in an automobile, of course, is a steering column shaft. However, the stresses on the magnetostrictive ring can be quite high, and none of the above-described magnetostrictive materials provide a desirable combination of mechanical strength and large magnetostriction. Accordingly, there remains a need for the development of materials suitable for such applications.